The present invention relates to methods of and apparatuses for warming coking drums in petroleum coking systems.
Coking systems are commonly used in petroleum refineries for converting vacuum tower bottoms and/or other heavy (i.e., high boiling point) residual petroleum materials to petroleum coke and other products. The greater part of each barrel of resid material processed in the coker will typically be recovered as fuel gas, coker gasoline/naphtha, light cycle oil (also commonly referred to by various other names such as light coker gas oil), and heavy cycle oil (also commonly referred to by various other names such as heavy coker gas oil).
A typical delayed coking system comprises: a combination tower or other fractionator; a fired heater; and at least one vertical coking drum. Most coking systems include at least a pair of vertical coking drums. The heavy coker feed is typically delivered to the bottom of the fractionator wherein it is combined with a heavy, liquid, residual bottom product (commonly referred to as a xe2x80x9crecyclexe2x80x9d) produced in the fractionator. The resulting mixture is drawn from the bottom of the fractionator and then pumped through the heater and into at least one coking drum. Typically, multiple coking drums are operated in alternating cycles such that, while one drum (referred to herein as the xe2x80x9clivexe2x80x9d drum) is -operating in a fill cycle, another drum is operating in a second cycle typically comprising a steaming stage, a cooling/quenching stage, a hydraulic decoking stage, a pressure testing stage, and a warm-up stage. During the warm-up stage, a portion of the product vapor produced in the live drum is used to warm the empty drum.
In the fill cycle, the hot feed material from the coker heater typically flows into the bottom of the live coking drum. Some of the heavy feed material vaporizes in the heater such that the material entering the bottom of the coking drum is a vapor/liquid mixture. The vapor portion of the mixture undergoes mild cracking in the coking heater and experiences further cracking as it passes upwardly through the coking drum. The hot liquid material undergoes intensive thermal cracking and polymerization in the coking drum such that the liquid material is converted to cracked vapor and petroleum coke. The resulting combined overhead vapor product produced in the coking drum is typically delivered to the fractionator wherein it is separated into gas, naphtha, light cycle oil, and heavy cycle oil, which are withdrawn from the fractionator as products, and the heavy recycle/residual material which flows to the bottom of the fractionator. The light and heavy cycle oil products are typically taken from the fractionator as side-draw products which are further processed (e.g., in a fluid catalytic cracker) to produce gasoline and other desirable end products. The heavy recycle material combines with the heavy feed material in the bottom of the fractionator and, as mentioned above, is pumped with the heavy feed material through the coker heater.
By way of example, but not by way of limitation, typical coker operating conditions and product specifications include: a heater outlet temperature in the range of from about 905 to about 935xc2x0 F.; coke drum pressures in the range of from about 20 to about 40 psig; live drum overhead temperatures in the range of from about 800xc2x0 to about 820xc2x0 F.; a fractionator overhead pressure in the range of from about 10 to about 30 psig; a fractionator bottom temperature in the range of from about 750xc2x0 to about 780xc2x0 F.; a light cycle oil draw temperature in the range of from about 450xc2x0 to about 550xc2x0 F.; a light cycle oil initial boiling point (ASTM D-1186) in the range of from about 300xc2x0 to about 325xc2x0 F.; a light cycle oil end point (D-1186) in the range of from about 600xc2x0 to about 650xc2x0 F.; a heavy cycle oil draw temperature in the range of from about 600xc2x0 to about 690xc2x0 F.; a heavy cycle oil initial boiling point (D-1186) in the range of from about 470xc2x0 to about 500xc2x0 F.; and a heavy cycle oil end point (D-1186) in the range of from about 960xc2x0 to about 990xc2x0 F.
One of the most serious and commonly encountered problems in delayed coking operations is foamover. Foamover typically results from the formation of an excessive volume of foam in the live coking drum during the fill cycle. When foamover occurs, partially coked resid is carried into the coke drum overhead line and, depending on the amount of such overflow, can result in: coke lay-down in the coke drum overhead lines; partial plugging of the combination tower bottoms screen; complete plugging of the combination tower bottom screen and a resultant sudden loss of feed to the coker heater; plugged (i.e., coked) heater tubes resulting from the sudden loss of flow therethrough; and plugging of the coker blowdown system. A massive foamover can even carry coke into the upper portions of the combination tower.
Foam is primarily formed from waxy, paraffinic condensed hydrocarbons present in the live coking drum during the fill cycle. A primary source of such material comprises condensate which forms in the live drum when hot resid is first switched into the drum at the beginning of the fill cycle. Although the empty drum is warmed prior to beginning the fill cycle, the warmed drum will typically still be very cool compared to the hot resid material flowing from the coker heater. Thus, some of the vapor condenses, particularly on the interior surface of the coking drum.
Each barrel of the condensed hydrocarbon material can form up to 1,200 barrels of foam in the live drum. The foam material travels up the coking drum on top of the coke layer.
Several factors promote the formation and expansion of foam material within the filling drum. These include: the amount of condensed and/or entrained liquid hydrocarbon material present in the drum; pressure swings in the live coking drum; a significant drop in overhead vapor product temperature; failure of the anti-foam chemical addition system; and over-filling the live drum. Particularly significant pressure losses typically occur in the live drum when a portion of the vapor product therefrom is diverted to warm up an empty drum.
Besides causing foam problems, condensate remaining or formed in the drum during the fill cycle detrimentally affects the quality and value of the coke product. Coke containing a significant amount of condensate material is commonly referred to as xe2x80x9cstickyxe2x80x9d or xe2x80x9cgreenxe2x80x9d coke.
The procedures heretofore used in the art for preventing foamovers have commonly included: attempting to ensure that the unit operators drain completely the warm, empty coking drum before beginning the fill cycle; injecting silicone anti-foam chemicals when a high foam level is detected in the live drum; restricting the fill rate so that the final level of the coke product is significantly below the top of the coking drum; and significantly limiting the amount of warm-up vapor taken from the live drum.
These approaches for reducing foam formation and expansion have serious shortcomings and are typically highly susceptible to operator error. Restricting fill rates and product levels significantly reduces unit capacity and, by necessitating the use of larger drums and/or a greater number of drums to achieve a given capacity, significantly increases construction costs. Silicone anti-foam chemicals are costly, unreliable, and can significantly poison catalysts used in fluid catalytic crackers and other downstream processing systems.
Most significantly, attempting to maintain live drum pressure by reducing the amount of warm-up vapor taken from the live drum can result in the empty drum being not sufficiently warmed before beginning the fill cycle. Foam formation rates increase rapidly with decreasing switchover temperatures, particularly below 600xc2x0 F. Unfortunately, however, due to the inadequacy of the warm-up procedures heretofore used in the art, switchover temperatures of as little as 450xc2x0 F. or less are common. Low switchover temperatures can also produce large pressure and temperature losses at the beginning of the fill cycle.
Due to the fact that the coking drum product vapor constitutes the primary feed to the coking unit fractionator, the fractionator is also adversely affected by pressure, temperature, and product flow fluctuations in the coking drum. Such changes can easily upset the operation of the fractionator and thus have a deleterious effect on the consistency and quality of the product fractions drawn from the fractionator.
In addition to these problems, a need also exists for a cost effective and efficient approach for significantly reducing warm-up vapor temperature and pressure losses during the drum warming process. In the coking units heretofore known in the art, the design and structure of the drum overhead systems have focused almost exclusively on the delivery of cracked vapor from the coking drums to the fractionator. It has been the practice of those skilled in the art to then conduct the drum warming procedure by diverting a portion of the product vapor from the live drum (i.e., the warm-up vapor) such that it simply travels in reverse flow through the overhead system to the top of the cold drum. As a result, the warm-up vapor has been required to travel a total of more than (typically much more than) 250 feet from the top of the live drum to the top of the cold drum. Even with the use of highly insulated piping, the temperature of the warm-up vapor will commonly decrease by as much as 90xc2x0 F. or more by the time it reaches the cold drum. Moreover, as a result of such temperature loss coupled with frictional losses and condensation, the motive pressure of the warm-up vapor will commonly decline by as much as 10 psi or more by the time it reaches the cold drum. These losses in temperature and motive pressure undesirably prolong the warming process and significantly reduce the switchover temperatures obtainable.
The present invention satisfies the needs and alleviates the problems discussed hereinabove by providing an improved method of producing petroleum coke in a petroleum coking system of the type having at least a pair of coking drums operating in a cyclical manner such that, when a first coking drum is being operated in a fill cycle wherein a coker feed material flows into the first coking drum to form coke and produce a vapor product, a second coking drum is operated in a second cycle including a warning stage wherein a flow of warm-up vapor comprising a portion of the vapor product is delivered through the second coking drum.
In one aspect, the inventive improvement comprises the steps of: (a) adding a fluid to the first coking drum during the warming stage effective for providing additional vapor in the first coking drum, the fluid being a material other than the coker feed material and (b) reducing pressure loss in the first coking drum during the warming stage by regulating the addition of the fluid to the first coking drum based upon a target pressure for the first coking drum and by regulating the flow of warm-up vapor from the first coking drum to the second coking drum based upon a target pressure for the second coking drum. In a particularly preferred embodiment of the invention, an actual pressure is determined for the first coking drum and the target pressure in the second coking drum is said actual pressure minus a targeted pressure differential.
In another aspect of the inventive method, the first coking drum has an upper end portion, the second coking drum has an upper end portion, and the improvement comprises conducting the flow of warm-up vapor from the upper end portion of the first coking drum to the upper end portion of the second coking drum via conduit means having a total length of less than 100 feet. Such conduit means will preferably have a total length of less than 75 feet.